Nd—Fe—B system permanent magnets have a growing range of application due to their excellent magnetic properties. While electronic equipment having magnets built therein including computer-related equipment, hard disk drives, CD players, DVD players, and mobile phones are currently under the trend toward size and weight reductions, higher performance and energy saving, there exists a demand to enhance the performance of Nd—Fe—B magnets, especially compact or thin Nd—Fe—B sintered magnets.
Indexes for the performance of magnets include remanence (or residual magnetic flux density) and coercive force. An increase in the remanence of Nd—Fe—B sintered magnets can be achieved by increasing the volume factor of Nd2Fe14B compound and improving the crystal orientation. To this end, a number of modifications have been made on the process. With respect to the increased coercive force, among different approaches including grain refinement, the use of alloy compositions with greater Nd contents, and the addition of effective elements, the currently most common approach is to use alloy compositions having Dy or Tb substituted for part of Nd. Substituting these elements for Nd in the Nd2Fe14B compound increases both the anisotropic magnetic field and the coercive force of the compound. The substitution with Dy or Tb, on the other hand, reduces the saturation magnetic polarization of the compound. Therefore, as long as the above approach is taken to increase coercive force, a loss of remanence is unavoidable.
In Nd—Fe—B magnets, the coercive force is given by the magnitude of an external magnetic field which creates nuclei of reverse magnetic domains at grain boundaries. Formation of nuclei of reverse magnetic domains is largely dictated by the structure of the grain boundary in such a manner that any disorder of grain structure in proximity to the boundary invites a disturbance of magnetic structure, helping formation of reverse magnetic domains. It is generally believed that a magnetic structure extending from the grain boundary to a depth of about 5 nm contributes to an increase of coercive force (See non-patent reference 1). The inventors found that by concentrating trace Dy or Tb only in proximity to the grain boundaries to increase the anisotropic magnetic field only in proximity to the boundaries, the coercive force can be increased while suppressing significant decline of remanence (see patent reference 1). Subsequently, the inventors established a production method comprising separately preparing a Nd2Fe1O compound composition alloy and a Dy or Tb-rich alloy, mixing them and sintering the mixture (see patent reference 2). In this method, the Dy or Tb-rich alloy becomes a liquid phase during the sintering and is distributed so as to surround the Nd2Fe14B compound. As a consequence, substitution of Dy or Tb for Nd occurs only in proximity to grain boundaries in the compound, so that the coercive force can be effectively increased while suppressing significant decline of remanence.
However, since the two types of alloy fine powders in the mixed state are sintered at a temperature as high as 1,000 to 1,100° C., the above-described method has a likelihood that Dy or Tb diffuses not only to the boundaries, but also into the interior of Nd2Fe14B grains. An observation of the microstructure of an actually produced magnet shows that Dy or Tb has diffused to a depth of about 1 to 2 μm from the boundary in a grain boundary surface layer, the diffused area reaching 60% or more when calculated as volume fraction. As the distance of diffusion into grains becomes longer, the concentration of Dy or Tb near the boundaries becomes lower. To positively suppress the excessive diffusion into grains, lowering the sintering temperature may be effective, but this measure cannot be practically acceptable because it compromises densification by sintering. An alternative method of sintering at lower temperatures while applying stresses by means of a hot press or the like enables densification, but poses the problem of extremely reduced productivity.
On the other hand, it is reported for small magnets that coercive force can be increased by applying Dy or Tb on the magnet surface by sputtering, and heat treating the magnet at a temperature lower than the sintering temperature, thereby causing Dy or Tb to diffuse only to grain boundaries (see non-patent references 2 and 3). This method allows for more effective concentration of Dy or Tb at the grain boundary and succeeds in increasing the coercive force without a substantial loss of remanence. As the magnet becomes larger in specific surface area, that is, the magnet form becomes smaller, the amount of Dy or Tb fed becomes larger, indicating that this method is applicable to only compact or thin magnets. However, there is still left the problem of poor productivity associated with the deposition of metal coating by sputtering or the like.    Patent reference 1: JP-B 5-31807    Patent reference 2: JP-A 5-21218    Non-patent reference 1: K. D. Durst and H. Kronmuller, “THE COERCIVE FIELD OF SINTERED AND MELT-SPUN NdFeB MAGNETS,” Journal of Magnetism and Magnetic Materials, 68 (1987), 63-75    Non-patent reference 2: K. T. Park, K. Hiraga and M. Sagawa, “Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Nd—Fe—B Sintered Magnets,” Proceedings of the Sixteen International Workshop on Rare Earth Magnets and Their Applications, Sendai, p. 257 (2000)    Non-patent reference 3: K. Machida, H. Kawasaki, M. Ito and T. Horikawa, “Grain Boundary Tailoring of Nd—Fe—B Sintered Magnets and Their Magnetic Properties,” Proceedings of the 2004 Spring Meeting of the Powder & Powder Metallurgy Society, p. 202